In such pump station arrangements the flow rate of the liquid is in general in the range of 0.7-1 meter per second, entailing the presence of a large liquid flow having a large momentum in the conduit extending from the pump. The flow rate of the liquid is usually higher than 0.7 meter per second in order to avoid sedimentation in the conduit and does not usually exceed 1 meter per second in order not to experience to high friction losses. Thereto the conduit may be thousands of meters long. If the supply of liquid from the pump to the conduit is abruptly stopped a pressure wave in the liquid will be generated that is transported through the pipe system and thereby different parts of the liquid will have different speed. This cumbersome situation may entail the generation of vacuum bubbles in the conduit and when these implode, e.g. different parts of the liquid moving in different directions in the conduit, so-called water hammer will occur that risk damaging the conduit and its units. Thereto, when the liquid column turn back towards the pump the conventional non-return valve that is located downstream the pump will slam shut and risk to become damaged.
In order to reduce the water hammer effects it is traditionally known to ramp down the frequency of the pump from the operational frequency to zero due to an automatically of manually generated turn off instruction. The purpose of ramping down is to have the pump to generate positive pump pressure all the time and thereby keeping the non-return valve open such that the flow rate of the liquid is lowered slowly such that no vacuum bubbles are generated in the conduit. In order to entirely eliminate vacuum bubbles the ramping down has to be very long, consuming unnecessary amounts of energy.
Thereto ramping down from the operational frequency to zero entail in reality that the non-return valve will become closed despite the impeller is still driven to generate a liquid flow but the pumping pressure and/or the liquid flow is too small to manage to pump liquid into the conduit. E.g. the pump consumes energy without performing any useful output.